The specification WO 02/47441 (Hu) has disclosed an abovementioned circuit arrangement. This specification has disclosed two PFC techniques: a charge pump (referred to as “single feedback” in the specification WO 02/47441 (Hu)) and a so-called valley-fill circuit (referred to as “double pump” in the specification WO 02/47441 (Hu)). In this case, the charge pump is connected upstream of the valley-fill circuit, when viewed from the system voltage. In addition, the charge pump and the valley-fill circuit are fed by the same high-frequency voltage source.
Charge pumps are known, for example, from the specification U.S. Pat. No. 4,949,013 (Zuchtriegel). A critical feature of a charge pump is the connection of a rectifier output to a pump diode. A high-frequency voltage, which is drawn from a load circuit, is applied to the resultant junction point. It should be emphasized at this point that only one rectifier output is connected to a pump diode, for which reason this part of the circuit arrangement is referred to as “single feedback” in WO 02/47441 (Hu).
Valley-fill circuits are known, for example, from the specification WO 90/09087 (Skalak). Of concern here is a passive PFC circuit comprising two storage capacitors and three diodes, which are connected between two rectifier outputs. The operation of the valley-fill circuit is based on the fact that the storage capacitors are connected via the diodes such that they are charged as a series circuit by a system voltage, but are discharged as a parallel circuit by a load.
A valley-fill circuit may also act as a charge pump by one of its diodes being split into two diodes. For this purpose, a high-frequency AC voltage is to be applied to the junction point of the two diodes produced by splitting. The specification U.S. Pat. No. 6,316,883 (Cho) has disclosed a valley-fill circuit which has been modified in this manner. The operating device described there for discharge lamps also has, in addition, a separate charge pump. This charge pump is connected downstream of the valley-fill circuit, when viewed from the system-voltage side, as a result of which a further storage capacitor is required.
In WO 02/47441 (Hu), the charge pump is connected upstream of the valley-fill circuit. A further storage capacitor is thus not required. The high-frequency AC voltage, which is fed into the charge pump, is derived from the high-frequency AC voltage which is fed to the modified valley-fill circuit.
It can be seen from the specification WO 02/47441 (Hu) that good values can be achieved for the power factor using the circuit arrangement described there. Standards such as IEC 61000-3-2 also prescribe, however, limit values for line current harmonics. In this case, a distinction is drawn between light sources which draw up to 25 W of power from the system voltage and light sources which draw more than 25 W. Above 25 W, the requirements are substantially higher, i.e. the amplitudes of the line current harmonics need to be substantially lower.
One subject matter of the specification WO 02/47441 (Hu) is a compact fluorescent lamp having an integrated operating device. Such lamps are conventional on the market up to a power consumed from the power supply system of 25 W. Since the requirements of relevant standards for the line current harmonics are so low up to 25 W, the circuit arrangement disclosed in WO 02/47441 (Hu) up to 25 W can provide for operation of fluorescent lamps which conforms with the standards.
The requirements for a circuit arrangement for operating light sources are diverse. The following requirements should be taken into account when designing these circuit arrangements:                high system-side power factor        low total harmonic distortion (THD) of the current consumed from the power supply system        line current harmonics which conform to standards        high degree of efficiency        low crest factor of the current through the light source        low radio interference        low costs        small geometric dimensions        
In order to operate fluorescent lamps having a power consumed from the power supply system of up to 25 W, the circuit arrangement disclosed in WO 02/47441 (Hu) represents a good compromise for meeting the abovementioned requirements. Above 25 W, however, it is problematic to adhere to the relevant standards for the line current harmonics. In particular for fluorescent lamps, the crest factor of the lamp current is limited by standards (for example IEC 60929) to a maximum value of 1.7. The adherence to this limit value at a power consumed from the power supply system of above 25 W is an additional problem.
Dimensioning the circuit arrangement disclosed in WO 02/47441 (Hu) to such an extent that the standards as regards line current harmonics are adhered to even in the case of a power consumed from the power supply system of above 25 W leads to components of the circuit arrangement being subjected to a considerably greater load. This leads to an increase in costs, to greater geometric dimensions and to a reduced degree of efficiency.
If, in addition, the limit value for the crest value of the lamp current in accordance with IEC 60929 is intended to be adhered to, the components will be subjected to an even greater load.
A circuit arrangement for operating light sources is known from the as yet unpublished DE 102004001617.8 which has, in addition to the features which are known from WO 02/47441 (Hu), a charge pump which is connected to the negative rectifier output. With such a circuit arrangement, a charge pump is accordingly connected both to the positive and the negative rectifier output. Even at a power consumed from the power supply system of above 25 W, line current harmonics which conform to standards and a lamp current crest factor which conforms to standards are thus achieved with components being subjected to only slight additional loads.
Finally, reference is made to the as yet likewise unpublished DE 102004001618.6 which likewise relates to a development of a circuit arrangement known from WO 02/47441 (Hu) and has the object of making possible a lamp current crest factor which conforms to standards even at a power consumed from the power supply system of above 25 W. This is achieved by a circuit arrangement which, in addition to the features which are known from WO 02/47441 (Hu), has a second resonant capacitor which does not have a pump action. A first resonant capacitor with a pump action and a second resonant capacitor without a pump action provide a degree of freedom with which the properties of the circuit arrangement can be better optimized.
It has been shown that, in the case of certain lamps, in particular HE lamps (HE=high efficiency), sufficiently low lamp current crest factors cannot be achieved despite these measures. Predominantly in the case of low temperatures, there is even the risk of unstable stroboscope-like lamp operation.